(1) Field of the Invention
This invention relates to radiographic apparatus of the non-CT (Computed Tomography) type used in medical, industrial and other fields for producing sectional images of patients or objects under examination. More particularly, the invention relates to a technique for shortening a processing time needed for image reconstruction and reducing artifacts appearing in the sectional images.
(2) Description of the Related Art
Conventional radiographic apparatus include an X-ray radiographic apparatus, for example. The X-ray radiographic apparatus has an X-ray tube and an image intensifier opposed to each other across an object under examination. The X-ray tube is linearly moved in a first direction, and in synchronism therewith the image intensifier is moved in a second direction counter to the first direction. With this movement, the apparatus intermittently performs radiography while varying an angle of X-ray emission from the X-ray tube to the object, such that a given point in a particular sectional plane of the object always corresponds to the same location on the detecting plane of the image intensifier. Then, a process is carried out simply to add detection signals so as to overlap a plurality of projected images acquired by radiography done from varied angles. In this way, the apparatus derives image information on a particular section of the object and image information on adjacent sections at opposite sides of the particular section.
Thus, the above X-ray radiographic apparatus is based the non-CT type radiographic technique distinct from the X-ray CT type radiographic technique which has made remarkable progress in recent years. That is, the X-ray CT type radiographic technique acquires transmitted images by driving an X-ray tube and an image intensifier opposed to each other across an object under examination to make one revolution (at least a half revolution) about the body axis of the object. An image reconstruction is carried out based on transmitted images acquired from one revolution (at least a half revolution) about the body axis of the object, to produce a sectional image seen in a direction along the body axis of the object. The non-CT type radiographic technique, as does the foregoing X-ray radiographic apparatus, produces a sectional image seen in a direction along the body axis of the object, without causing the X-ray tube and image intensifier to make a half or more revolution about the body axis of the object.
Nowadays, further improvement is desired for the non-CT type radiographic technique, and in this context, a possibility of employing what is known as a back projection method is being explored. This method does not perform an image reconstruction to obtain two-dimensional slice image data by adding detection signals so as to overlap a plurality of projected images acquired by radiography done from varied angles. Instead, an image reconstruction is carried out to produce three-dimensional volume data of a region of interest by projecting a plurality of projection images obtained by radiographing the region of interest from varied angles, back to predetermined lattice points on a three-dimensional lattice virtually set to the region of interest of the radiographed object. The back projection method of the non-CT type radiographic technique can acquire three-dimensional volume data of a region of interest of an object in one radiographic operation. This provides an advantage of enabling a particular sectional image to be selected and displayed promptly after the radiographic operation.
However, the conventional technique noted above has the following drawback. The back projection method of the non-CT type radiographic technique carries out an image reconstruction to produce three-dimensional volume data of a region of interest by projecting a plurality of projection images obtained by radiographing the region of interest from varied angles, back to predetermined lattice points on a three-dimensional lattice virtually set to the region of interest of the radiographed object. The number of lattice points on the three-dimensional lattice, for example, corresponds to the tube of the number (100 to 1,000) of points equally arranged along each of the three axes. A projection image obtained from a particular angle, i.e. data detected by pixels on the detecting plane of the image intensifier, is projected back to predetermined lattice points on the three-dimensional lattice. This is done for a plurality of projection images obtained from varied angles. Thus, an enormous volume of data is back-projected, resulting in an extended processing time for the image reconstruction to generate three-dimensional volume data of the region of interest.
This invention has been made having regard to the state of the art noted above, and its object is to provide a radiographic apparatus which requires a reduced processing time for image reconstruction.
To fulfill the above object, Inventor has made intensive research and attained the following findings. The non-CT type radiographic technique is characterized in that a radiation source such as an X-ray tube and an area detector such as an image intensifier are not driven to make more than a half revolution about the body axis of an object under examination. To illustrate a radiographed region of interest of the object with a three-dimensional coordinate system, only a small amount of information is collected for the direction of an axis extending substantially through the center of the region of interest and perpendicular to a sectional plane. Resolution is thus lower in this direction than in the directions along the two remaining axes (which are within the sectional plane). Consequently, lattice spacing along the axis extending through the slice plane may be made larger than the lattice spacing along the other axes of a three-dimensional lattice virtually set to the region of interest of the object, without wasting image information for the direction of this axis. Rather, the processing time may be shortened.
Based on the above findings, this invention provides a radiographic apparatus for generating three-dimensional volume data of a region of interest of an object under examination by an image reconstruction of projection data acquired by radiographing the object from varied scan positions, and obtaining sectional images from the three-dimensional volume data, the apparatus comprising:
a radiation source for irradiating the object with penetrating electromagnetic waves;
an area detector for detecting electromagnetic waves transmitted through the object;
the radiation source and the area detector being arranged across sectional planes of the object and synchronously operable for scanning action; and
a back projection unit for performing the image reconstruction to generate three-dimensional volume data of the region of interest by projecting projection data detected in the varied scan positions back to predetermined lattice points of a three-dimensional lattice virtually set to the region of interest of the object radiographed;
the back projection unit generating the three-dimensional volume data, with lattice spacing along a sectional axis extending substantially through the center of the region of interest and perpendicular to the sectional planes, among three orthogonal axes of the three-dimensional lattice, made larger than lattice spacing in the two other directions.
With the apparatus according to this invention, a three-dimensional lattice is virtually set to the region of interest of the object radiographed. The back projection unit increases the lattice spacing along a sectional axis extending substantially through the center of the region of interest and perpendicular to the sectional planes, among three orthogonal axes of the three dimensional lattice, to be larger than the lattice spacing in the two other directions. Then, the back projection unit performs an image reconstruction to generate three-dimensional volume data of the region of interest by projecting projection data detected in varied scan positions back to predetermined lattice points of the three-dimensional lattice having the enlarged lattice spacing along the sectional axis. Thus, compared with a back projection to a conventional three-dimensional lattice having an equal lattice spacing along the three axes, the data back-projected may be reduced by an amount corresponding to the enlarged lattice spacing along the sectional axis of three-dimensional lattice over the lattice spacing in the two other directions. The processing time relating to the image reconstruction may be shortened accordingly.
Preferably, the lattice spacing along the sectional axis has a length set based on a detection pixel length along the sectional axis which is a length of one pixel of the area detector projected to the sectional axis. This feature allows the length of the lattice spacing along the sectional axis to be set according to what is known as a lamino angle which is an angle between the sectional axis and a straight line extending from the radiation source to the center of the detecting plane of the area detector.
Preferably, a low-pass filtering unit is provided for applying a low-pass filter in a direction along the sectional axis of the projection data detected in the varied scan positions. This results in sectional images with reduced artifacts due to the influence of missing cones.
Preferably, the low-pass filtering unit includes a three-dimensional Fourier transform unit for performing a three-dimensional Fourier transform of the three-dimensional volume data generated by the back projection unit, a Fourier space low-pass filtering unit for applying a low-pass filter along the sectional axis of the Fourier space data after the three-dimensional Fourier transform, and a three-dimensional back Fourier transform unit for performing a three-dimensional back Fourier transform of the Fourier space data after application of the low-pass filter and putting the Fourier space data back to three-dimensional volume data. This further promotes the generation of sectional images with reduced artifacts due to the influence of missing cones.
Preferably, the low-pass filtering unit applies a low-pass filter for diffusing at least four times a detection pixel length along the sectional axis. This feature sufficiently suppresses artifacts in an F space filter method and a 2-D filtering method.
Preferably, high frequency components along the sectional axis of the Fourier space data to which the low-pass filter has been applied are cut, thereby generating a reduced number of sectional images of subsequent three-dimensional volume data undergoing the three-dimensional back Fourier transform by the three-dimensional back Fourier transform unit. The reduced volume of image information in the direction along the sectional axis correspondingly reduces the processing time for the three-dimensional back Fourier transform, and the number of sectional images in the direction along the sectional axis. This is effective where there is no need to reproduce images at small intervals along the sectional axis.
Preferably, the area detector is a flat panel detector having gate lines arranged in a direction along the sectional axis, the low-pass filtering unit performing low-pass filtering by simultaneously turning on gates on a predetermined number of pixel lines corresponding to the direction along the sectional axis. Such low-pass filtering based on the simultaneous actuation of the gates also is effective to produce sectional images with reduced artifacts due to the influence of missing cones.
Preferably, a real space low-pass filtering unit is provided for applying a low-pass filter along the sectional axis corresponding to predetermined times a detection pixel length along the sectional axis, to predetermined numbers of pixels of the area detector corresponding to the sectional axis. This feature also is effective to produce sectional images with reduced artifacts due to the influence of missing cones.
Preferably, the back projection unit is arranged project the projection data after the low-pass filtering back to the three-dimensional lattice, with the lattice spacing along the sectional axis of the three-dimensional lattice set between a detection pixel length along the sectional axis and a diffusion length along the sectional axis. The three-dimensional back Fourier transform is performed to generate the three-dimensional volume data diffused along the sectional axis by reducing the image information in the direction along the sectional axis. This reduces the processing time for the three-dimensional back Fourier transform, and the number of sectional images in the direction along the sectional axis. This is effective where there is no need to reproduce images at small intervals along the sectional axis.